1. Field of Invention
This invention relates generally to digital signal processing and specifically to controlling the volume of a DVD player.
2. Description of Related Art
FIG. 1 shows a conventional digital sound system 1 configured in accordance with the MPEG2 standard, where the left and right channels of an incoming music signal S are coupled into a receiving circuit 10. The resultant left and right channel samples are combined in a well known manner and provided as a stereo signal to a first input terminal of a multiplier 12. A volume signal VOL provided by a volume control knob (not shown) is coupled to a second input terminal of the multiplier 12. The multiplier 12 multiplies the input stereo signal and the input volume signal to produce a volume adjusted, output stereo signal. Here, volume control of the stereo signal is realized by shifting bits of the stereo signal in response to the volume signal. The volume adjusted stereo signal is provided to a memory 14 for buffering, and thereafter converted to an analog stereo signal using a digital-to-analog converter (DAC) 16. The resultant analog stereo signal is coupled to a first input terminal of an analog mixing circuit 18. The mixer 18 includes a second input terminal coupled to receive an analog microphone input signal MIC provided by an associated microphone (not shown). In response to a control signal C received at its control terminal, the mixer 18 provides to a loudspeaker 20 either the analog stereo signal or the analog microphone signal superimposed onto the analog stereo signal.
Although the volume control technique mentioned above is relatively simple to implement, instantaneously changing the volume of a stereo signal in such a manner often results in an audible xe2x80x9cpoppingxe2x80x9d noise. If the volume is set to a sufficiently high level, this popping noise may blow the attached speakers. One solution offered to eliminate the popping noise is to mute the output stereo signal during volume transitions. However, the resultant silence introduced into the output signal during volume transitions is unacceptable to some listeners. Further, conventional channel muting techniques such as, for instance, disabling the DAC or zeroing the stereo samples while buffered in memory, requires complex logic circuitry which, in turn, undesirably introduces additional timing considerations and consumes valuable silicon area. Thus, there is a need for an improved audio signal interface which alleviates the above-de,scribed problems.
An audio interface is disclosed which eliminates popping noise during volume transitions and implements a channel muting function while saving silicon area. In accordance with the present invention, an audio interface is coupled to receive a music signal and a microphone signal. The music signal and a volume control signal are combined in a multiplier to produce a volume adjusted music signal. In response to an input signal from a user, the volume control signal is gradually changed in predetermined increment levels. The resulting music and microphone signal samples are stored in corresponding partitions of a single memory, and thereafter provided to a mixing circuit. The mixing circuit combines signal samples read from the memory to produce four output signals each containing first and second channel samples. The resultant 8 channel samples are gated in a formatter with respective channel mute signals which, when asserted, effectively mute their corresponding channel samples.